Aircraft floor and interior panels using edge coated honeycomb

ABSTRACT

Aircraft floor and interior panels where core-skin bonding is improved between honeycomb and composite face sheets (skins) by applying a polyamide and/or rubber-containing adhesive to the edge of the honeycomb prior to bonding. Edge coating of the honeycomb allows one to reduce panel weight without reducing the performance parameters that are required for different types of aircraft floor and interior panels.

This application is a continuation-in-part of application Ser. No.11/228,601 filed on Sep. 16, 2005 now U.S. Pat. No. 7,581,366, which isa continuation-in-part of application Ser. No. 10/932,510, filed on Sep.1, 2004, now U.S. Pat. No. 7,507,461.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to sandwich panels and otherrelated structural composite materials. Sandwich panels are typicallymade up of face sheets (also referred to as “skins”) that are adhesivelybonded to opposite sides of a core material, such as honeycomb, to formthe sandwich panel. In particular, the present invention relates totreating honeycomb in order to improve the bond between the honeycomband face sheets. The invention is particularly applicable to aircraftfloor and interior panels, which are composed of composite honeycomb andcomposite face sheets.

2. Description of Related Art

Sandwich panels are used in a wide variety of applications where highstrength and light weight are required. Honeycomb and rigid foam areused as the core in many sandwich panels. Honeycomb is a popular corematerial because of its high strength to weight ratio and resistance tofatigue failures. A wide variety of products including metals andcomposite materials have been used to make the honeycomb cores.

The face sheets that are bonded to the side (also referred to as the“edge”) of the honeycomb have also been made from a wide variety ofmaterials including metals and composites. An important consideration inthe formation of any sandwich panel is the way in which the face sheetsare bonded to the honeycomb. Typically, an adhesive is used to bond theface sheets to the core. The adhesive must rigidly attach the facings orskins to the core in order for loads to be transmitted from one facingto the other and to permit the structure to fulfill all the assumptionsimplied in the acceptance of the commonly used stress calculationmethods. If the adhesive fails, the strength of the panel is severelycompromised. The adhesive is especially critical in sandwich panelswhich use honeycomb as the core because of the relatively small surfacearea over which the edges of the honeycomb contact the face sheets.

One procedure for applying composite face sheets to honeycomb involvesforming a prepreg sheet that includes at least one fibrous reinforcementlayer and an uncured resin matrix. Prepreg is a term of art used in thecomposite materials industry to identify mat, fabric, non-wovenmaterial, tow or roving which has been pre-impregnated with resin andwhich is ready for final curing. A film adhesive is typically added tothe prepreg-core assembly and it is then bonded to the honeycomb bycuring of both the prepreg resin and adhesive resin at elevatedtemperature. The film adhesive can be applied as a separate ply layer oras an integral part of the prepreg sheet.

Honeycomb sandwich panels are used in many applications where stiffnessand structural strength of the panel are primary considerations.Additionally, honeycomb sandwich panels are also widely used in theaerospace industry where the weight of the panel is also of primaryimportance. As a result, there has been and continues to be a concertedeffort to reduce the weight of the honeycomb sandwich panels withoutsacrificing structural strength. One area which has been investigated toreduce weight is the elimination of separate adhesive layers. This hasbeen accomplished by making the face sheets from composite materialsthat are self-adhesive. The resins used in such self-adhesive prepregsmust meet the dual requirements of providing suitable structuralstrength while still providing adequate adhesion to the honeycomb.Exemplary self-adhesive face sheets are described in published EuropeanPatent Application Nos. EP0927737 A1 and EP0819723 A1 and in U.S. Pat.Nos. 6,440,257 and 6,508,910.

An alternative method of bonding face sheets to honeycomb involvesapplying an adhesive to the edge of the honeycomb. The adhesive istypically applied by “dipping” the edge of the honeycomb in theadhesive. The adhesives used in this type bonding are typically referredto as “dip” resins or adhesives. The advantage of this method is thatthe adhesive is located only where the honeycomb contacts the facesheet, rather than being distributed over the entire face sheet. Thismethod is generally used to bond non-adhesive face sheets, such asaluminum and other metallic face sheets, to the honeycomb.

Honeycomb sandwich panels have been used as floor panels in thefuselages of aircraft. Floor panels are used, especially in largecommercial aircraft, to separate the passenger compartment from thevarious electrical, hydraulic and structural components located in thebottom of the fuselage. The floor panels are generally classifieddepending on their location and intended use. For example, largecommercial aircraft typically include four different types of floorpanels: 1) aisle panels; 2) under seat panels; 3) galley panels; and 4)high load panels. The physical characteristics of the panels varydepending on the particular loads experienced during use. For example,the aisle and under seat floor panels tend to be lighter and not asstrong as galley and high load floor panels.

It is widely known that an important consideration in the design andconstruction of any aircraft is to keep overall weight at a minimum.Since the amount of floor panels present in a large commercial ormilitary aircraft can be on the order of 1000 to 3000 square feet andmore, it is important to make the floor panels as light as possiblewhile still providing the various structural properties that arerequired for the particular type of floor panel. Even a small change infloor panel density results in a relatively large change in the overallweight of the aircraft. Accordingly, there is a continuing need toprovide floor panels that are light in weight and which still meet thevarious structural requirements for use in aircraft and other aerospacevehicles.

There are also a large number of interior panels in aircraft that are inthe form of sandwich panels. These interior panels do not have to be asstructurally strong as floor panels. Such interior panels includesandwich panels that are used to form overhead storage compartments andother interior storage structures, interior non-structural bulkheads,sidewalls and other interior walls, ceiling panels and lavatory panels.The interior sandwich panels are typically lighter than floor panels.The panels must be sufficiently strong to meet required design loads foraircraft interior use while at the same time being as light as possible.In addition, interior panels must meet strict flammability standards.Accordingly, there is also a continuing need to provide interior panelsthat are light in weight and which meet the various requirements for usein aircraft and other aerospace vehicles

SUMMARY OF THE INVENTION

In accordance with the present invention, honeycomb sandwich panels areprovided that include at least one face sheet that has an interiorsurface that is bonded to a honeycomb core and an exterior surface. Theface sheet includes at least one fiber layer and a resin matrix. Thehoneycomb includes walls that define a plurality of honeycomb cellswherein the walls have at least one edge that is bonded to the interiorsurface of the face sheet. As a feature of the invention, an adhesivethat includes polyamide (a common example of which is commonly referredto as “nylon”) and/or rubber is used to bond the face sheet to thehoneycomb. It was discovered that the use of an adhesive that includespolyamide and/or rubber provides an especially strong bond between thehoneycomb and face sheets where the face sheet includes a resin matrix.The use of a polyamide and/or rubber-containing adhesive in accordancewith the present invention was found to be particularly useful inincreasing the bonding strength between honeycomb and prepreg facesheets.

As a further feature of the present invention, adhesives that includepolyamide and/or rubber provide an increase in the bond strength betweenhoneycomb and prepreg face sheets where the resin matrix used for theprepreg includes a thermosetting resin. The use of polyamide adhesivesis particularly useful in increasing the bond strength of self-adhesiveprepreg face sheets where the prepreg resin matrix includes a rubberand/or thermoplastic toughened thermosetting resin. In some embodimentsof the invention, the bond strength provided by the polyamide adhesiveis such that, during separation of the face sheet from the core (in drumpeel tests), the honeycomb wall is torn apart while the bond between theface sheet and the honeycomb remains intact. In these embodiments, theadhesive bond is stronger than the core so that the sandwich panelundergoes core failure rather than cohesive failure when the face sheetis peeled from the panel.

As an additional feature of the present invention, adhesives thatinclude polyamide are particularly well suited for bonding face sheetsto honeycomb that is made from composite materials. During thefabrication process, a honeycomb slice is typically cut from a largeblock of honeycomb. The slicing of the honeycomb from the block tends toexpose fibers and form “ragged” or “fuzzy” edges that tend to bedifficult to bond to the face sheet. Accordingly, in order to improvebond strength, the edges of composite honeycomb are generally treatedprior to bonding by sanding, machining or other surface treatment toremove the exposed fibers and smooth the honeycomb edge. The polyamideand/or rubber-containing adhesives in accordance with the presentinvention provide good bond strength when either fuzzy or smoothhoneycomb is used. Accordingly, the time consuming step of removing thefuzzy edges from the composite honeycomb may be deleted if desired.

The present invention not only covers sandwich panels as described abovethat are made using honeycomb which is edge-coated with a polyamideand/or rubber-containing adhesive, but also covers methods for bondingface sheets to honeycomb to form such sandwich panels. The methodsinvolve providing a face sheet that includes an interior surface forbonding to the honeycomb core and an exterior surface wherein the facesheet includes at least one fiber layer and a resin matrix. A honeycombis provided that includes walls that define a plurality of honeycombcells wherein the walls have at least one edge for bonding to theinterior surface of the face sheet. An adhesive that includes polyamideand/or rubber is applied to the honeycomb edge to provide an edge-coatedhoneycomb that is then bonded to the face sheet.

The finished sandwich panels made in accordance with the presentinvention may be used in a wide variety of situations where alightweight and structurally strong material is needed. However, theinvention is especially well suited for use in aerospace applicationswhere a multitude of strict mechanical and chemical requirements must bemet while at the same time not exceeding weight limitations. Forexample, the sandwich panels of the present invention are useful asfloor panels in relatively large aircraft where the amount of floor areain the fuselage can range from a few hundred square feet up to 3000square feet and more. The combination of low weight and structuralstrength provided by floor panels in accordance with the presentinvention is particularly desirable when such large amounts of floorpanels are used.

The sandwich panels in accordance with the present invention are alsouseful as interior panels in aircraft and other aerospace vehicles. Suchinterior panels are typically lighter than floor panels and generallymust meet strict flammability requirements. The panel face sheets aretypically made using flame resistant phenolic resins or thermoplasticsthat meet the flammability requirements for aircraft interiorstructures. It was discovered that the peel strength of interior panelsmade using such flame resistant materials is increased substantiallywhen the core edge is coated with a polyamide and/or rubber-containingadhesive.

The above-described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preferred exemplary honeycomb core andtwo composite face sheets prior to bonding of the honeycomb to the facesheets using an edge-coating adhesive in accordance with the presentinvention that contains polyamide or a modified polyamide and/or rubber.

FIG. 2 is a perspective view of a preferred exemplary sandwich panelwhere the components shown in FIG. 1 have been bonded together and curedto form the final honeycomb sandwich panel.

FIG. 3 is a partially diagrammatic representation of an exemplary methodin accordance with the present invention showing a single edge-coatedhoneycomb wall being bonded to a face sheet.

FIG. 4 is a partially diagrammatic view showing an embodiment of thepresent invention where the sandwich panel undergoes core failure duringforcible separation of the face sheet from the honeycomb.

FIG. 5 is a partial sectional view of an aircraft fuselage that showsexamples of various types of floor and interior panels, which aretypically present in a large aircraft.

DETAILED DESCRIPTION OF THE INVENTION

It is preferred that the nylon (polyamide) and/or rubber-containingadhesives in accordance with the present invention be used to improvethe bonding of composite face sheets (particularly self-adhesiveprepregs) to honeycomb cores to form light-weight structural panels foruse in aerospace applications where light weight and structural strengthare especially important design criteria. However, those of ordinaryskill will recognize that the present invention is not limited toaerospace applications, but may also be used in any situation wherethere is a need for honeycomb sandwich panels that have face sheets,which are made from composite materials.

The three basic components of a preferred exemplary honeycomb sandwichpanel for use in aerospace applications are shown in FIG. 1 prior toformation of the panel. The components include a honeycomb core 12 thathas walls 11 which form a plurality of honeycomb cells 13. The wallshave edges that form the faces or edges of the honeycomb as shown at 14and 16. The other two components are prepreg face sheets 17 and 19. Theface sheets 17 and 19 include interior surfaces 21 and 23, respectively,for bonding to the honeycomb edges. The face sheets 17 and 19 alsoinclude exterior surfaces 25 and 27, respectively. In accordance withthe present invention, the edges 14 and 16 of honeycomb 12 are coatedwith an adhesive (not visible in FIG. 1) that includes polyamide and/orrubber as an essential ingredient to form an edge-coated honeycomb. Theuse of an adhesive that contains polyamide and/or rubber in accordancewith the present invention improves the bonding of face sheets 17 and 19to the honeycomb 12, while adding very little to the weight of theresultant panel. The uncured prepreg face sheets or skins 17 and 19 areapplied to the edge-coated honeycomb 12 and then cured to form facesheets 18 and 20 of the finished panel 10 as shown in FIG. 2.

FIG. 3 is a partially diagrammatic representation showing the method forbonding the honeycomb to one of the face sheets 19 in accordance withthe present invention. For simplicity, FIG. 3 shows the bonding of asingle wall 11 with it being understood that the entire edge 16 of thehoneycomb is coated with the polyamide-based adhesive (coating). Asshown in FIG. 3, the lower edge of honeycomb wall 11 is coated with apolyamide and/or rubber-containing-based adhesive 15 prior to bonding tothe lower uncured prepreg face sheet 19. As represented by arrow 30, theedge-coated honeycomb is located against the face sheet 19 such that thepolyamide and/or rubber-containing adhesive flows partially onto theprepreg 19 to form fillets 28 and 29. Then, as represented by arrow 40,the prepreg is cured to form the final sandwich panel 10 that includesthe cured face sheet 20.

The honeycomb core 12 can be made from any of the materials which areused to form honeycomb cores. Exemplary honeycomb materials includealuminum, aramid, carbon or glass fiber composite materials, resinimpregnated papers, non-woven spun bonded polypropylene, spun bondednylon, spun bonded polyethyleneterephthlate (PET), and the like.Exemplary preferred honeycomb materials are aramid-based substrates,such as those marketed under the trade name NOMEX® which are availablefrom E.I. DuPont de Nemours & Company (Wilmington, Del.). Honeycombcores made from NOMEX® are available commercially from HexcelCorporation (Dublin, Calif.). Preferred exemplary NOMEX® honeycombsinclude HexWeb® HRH-10 which is available from Hexcel Corporation.Another preferred honeycomb material is resin impregnated aramid fibers,such as KEVLAR® fibers impregnated with phenolic resin. Preferredexemplary KEVLAR® honeycomb is available from Hexcel Corporation underthe trade name Hexweb® HRH-36. Honeycomb made from carbon or glasscomposites are also preferred and typically include carbon or glassfabric and a phenolic and/or polyimide matrix. The honeycomb istypically supplied in a cured form and requires no further treatmentprior to bonding to the skins or face sheets other than application ofthe polyamide and/or rubber-containing adhesive as described below. Ifdesired, the cores may be sanded or otherwise machined to remove the“fuzzy” edges that may be present due to fibers that are exposed duringslicing of the honeycomb from the larger block of honeycomb. However, asmentioned previously, the use of polyamide and/or rubber-containingadhesives in accordance with the present invention as an edge coatingeliminates or at least substantially reduces the need for removing thefibers that are exposed on the honeycomb edges during fabrication.

The dimensions of the honeycomb can be varied widely. For aerospace use,the honeycomb cores will typically have ⅛ to ½ inch (3.2-12.7 mm) cells(i.e., in diameter/cross-section) with the cores being ¼ inch (6.4 mm)to 2 inches (50.8 mm) thick (distance between the honeycomb edges). Thethickness of the honeycomb walls may also be varied with typicalhoneycomb walls being on the order of 0.001 inch (0.25 mm) to 0.005 inch(0.13 mm) thick. The combination of cell size, wall thickness anddensity of the material that is used determines the weight of the core,which is expressed in pounds per cubic foot (pcf). Composite honeycombhaving weights on the order of 1.5 pcf to 5.0 pcf are preferred foraircraft interior panels. For aircraft floor panels, honeycomb weight onthe order of 5.0 pcf to 15.0 pcf are preferred.

The face sheets that are bonded to the honeycomb can be made from any ofthe materials that are used in composite sandwich panel construction.Such composite materials usually include one or more layers of fibersand a resin matrix. The fibers that are used in the prepreg face sheets17 and 19 can be any of the fiber materials that are used to formcomposite laminates. Exemplary fiber materials include glass, aramid,carbon, ceramic and hybrids thereof. The fibers may be woven,unidirectional or in the form of random fiber mat. Woven carbon fibersare preferred, such as plain, harness satin, twill and basket weavestyles that have areal weights from 80-600 gsm, but more preferably from250-400 gsm. The carbon fibers can have from 3,000-40,000 filaments pertow, but more preferably 3,000-12,000 filaments per tow. All of whichare commercially available. Similar styles of glass fabric may also beused with the most common being 7781 at 303 gsm and 120 at 107 gsm. Whenunidirectional constructions are used, typical ply-weights are 150 gsmfor carbon and 250 gsm for glass.

Although the composite material may be pre-cured prior to bonding to thehoneycomb, it is preferred that resin matrix be in an uncured orpartially cured state prior to bonding. The use of uncured face sheetsis preferred because it combines curing and bonding of the face sheet tothe honeycomb into a single step and allows the polyamide and/or rubbercontaining adhesive present on the honeycomb to interact with theuncured resin matrix during the bonding process. Prepreg is a preferredform or type of uncured face sheet.

The resin matrix for the prepreg should include a thermosetting resinsuch as epoxy, cyanate ester, phenolic, benzoxazines, polyimide orbismaleimide. Prepregs that have some inherent adhesiveness(self-adhesive prepreg) are particularly preferred. Prepregs of the typedescribed in issued U.S. Pat. Nos. 6,440,257 and 6,508,910 are examplesof self-adhesive prepregs that are capable of bonding to honeycomb toform a suitable sandwich panels without the use of an adhesive layer.These prepregs include a resin matrix that is a combination ofthermosetting and thermoplastic resins. Use of nylon and/orrubber-containing adhesives to coat the honeycomb edges was found toprovide an unexpected increase in bond strength between the honeycomband these types of self-adhesive face sheets. Other exemplary facesheets having adhesive properties are available from J.D. Lincoln Inc.(Costa Mesa, Calif.) under the tradename L-591. L-591 is a modifiedphenolic prepreg that contains 39 percent by weight phenolic resin incombination with 7781-style fiberglass. CYCOM® 2290 available from CytecCorporation (Anaheim, Calif.) is another phenolic-based prepreg that issuitable for use as a face sheet. Face sheet that are made usingphenolic resins are preferred for use in aircraft interior panels.

The preferred resins that are used to form the resin matrix for sandwichpanels, other than aircraft interior panels, include epoxy and/orcyanate ester and/or bismalemide resins and one or more curing agents.The resin matrix also preferably includes viscosity control agents andthermoplastic fillet forming particles as described in U.S. Pat. Nos.6,440,257 and 6,508,910. When viscosity control agents and filletforming particles are included in the resin matrix, the resin(s) arefirst mixed with viscosity control agents to form a resin mixture. Ifnecessary, the mixture is heated to ensure that viscosity control agentsare completely dissolved. Curing agents and fillet forming particles arethen added to the resin mixture. This final resin mixture is kept belowthe temperature at which the fillet forming particles dissolve in theresin. As a result, the fillet forming particles, which at this stageare preferably uniformly mixed throughout the resin, are not dissolvedto a substantial degree and therefore do not increase the resinviscosity to an unacceptable level. The viscosity of the resin mixtureis important because it must be such that the resin can be impregnatedinto the fiber to form the prepreg. For the purposes of thisspecification, particles which retain at least 90 weight percent oftheir original particle weight are considered to be not dissolved to asubstantial degree. Particles are considered to be substantiallydissolved when less than 10 percent by weight of the original particleremains intact within the resin.

The viscosity of the final resin mixture that is used to make theprepreg, including fillet-forming particles, should be between 150 and1500 poise. The preferred viscosity is between 300 to 1200 poise. Thepreceding viscosity ranges represent minimum viscosities for the finalresin mixture prior to making prepreg when said viscosity is measured byRheometric Dynamic Analysis (Rheometrics RDA2) at settings of 2° C./min,10 rads/sec and 0.8-1.0 mm gap. The viscosity of the resin mixturegradually increases when the fillet forming particles dissolve duringthe curing/bonding process.

During fabrication of the prepreg, the fillet forming particles tend tobe concentrated toward the surface of the prepreg due to inherentfiltering of the particles by the fiber layer. Alternatively, the filletforming particles may be applied (e.g., by powder deposition) to thesurface of the resin after it has been formed into a prepreg film orafter the resin has been impregnated into the fiber layer. In this way,the fillet forming particles are distributed substantially on thesurface of the prepreg. In either case, the resin temperature ismaintained at a sufficiently low level to prevent the fillet formingparticles from dissolving until the prepreg is applied to the corematerial and cured.

After application to the edge-coated core, the prepreg is heated to asufficient level for a sufficient time to substantially dissolve thefillet forming particles, cure the prepreg and cure the polyamide-basedadhesive. The dissolved thermoplastic particles enhance the toughness ofthe bond and interact with the polyamide adhesive (as well as thethermosetting resin present in the prepreg matrix resin) to even furtherstrengthen the bond.

Exemplary thermosetting resins which may be used to make the prepregmatrix resin include phenolics, benzoxazines, polyimides, epoxy, cyanateester and bismaleimide resins. Exemplary epoxy and cyanate ester resinsinclude glycidylamine type epoxy resins, such astriglycidyl-p-aminophenol, tetraglycidyldiaminodiphenyl-methane;glycidyl ether type epoxy resins, such as bisphenol A type epoxy resins,bisphenol F type epoxy resins, bisphenol S type epoxy resins, phenolnovolac type epoxy resins, cresol novolac type epoxy resins (e.g. ECN1299 available from Huntsman Advanced Materials, Inc., The Woodlands,Tex.) and resorcinol type epoxy resins; cyanate esters, such as1,1′-bis(4-cyanatophenyl)ethane (AroCy L-10, available from HuntsmanAdvanced Materials, Inc., The Woodlands, Tex.),1,3-Bis(4-cyanateophenyl-1-1-(1-methylethylidene) benzene (RTX366,available from Huntsman Advanced Materials, Inc., The Woodlands, Tex.).Epoxy resins are preferred for sandwich panels other than aircraftinterior panels. Especially preferred epoxy blends include a mixture oftrifunctional epoxy and a difunctional bis-F epoxy.

Curing agents and viscosity control agents (optional) are also added tothe thermosetting resin to form the basic prepreg matrix resin. Thecuring agent is preferably an amine curing agent and the viscositycontrol agent is preferably a thermoplastic material that dissolves inthe thermosetting resin.

Although a wide variety of matrix resins may be used in the compositeface sheets, matrix resins based on epoxy and cyanate ester formulationsare preferred for aircraft floor panels and other applications where aflame resistant resin, such as a phenolic, is not required. In addition,sandwich panels that have two prepreg plies on each face of thehoneycomb are preferred. The two layers are preferably either two (0/90)plies or two (±45, 0/90) plies with warp direction aligned with thelengthwise direction of the honeycomb. Those of ordinary skill in theart will recognize that the present invention is also applicable tomultiple ply face sheets that include two or more fiber layers.

Exemplary preferred face sheet matrix resin formulations are as follows:

1) 1 to 70 parts by weight of an epoxy;

-   -   5 to 40 parts by weight of an amine curing agent;    -   1 to 30 parts by weight of a viscosity control agent; and    -   5 to 50 parts by weight of thermoplastic fillet forming        particles.

2) 10 to 40 parts by weight of a trifunctional epoxy resin;

-   -   10 to 40 parts by weight of a difunctional epoxy resin;    -   11 to 25 parts by weight of an aromatic curing agent;    -   0 to 3 parts by weight of a non-aromatic curing agent;    -   5 to 15 parts by weight of a viscosity control agent; and    -   8 to 30 parts by weight of thermoplastic fillet forming        particles.

The epoxy may be composed of trifunctional epoxy, difunctional epoxy anda wide variety of combinations of trifunctional and difunctionalepoxies. Tetrafunctional epoxies may also be used. Exemplarytrifunctional epoxy include triglycidyl p-aminophenol andN,N-Diglycidyl-4-glycidyloxyaniline (MY-0510 or MY-0500 available fromHuntsman Advanced Materials, Inc., The Woodlands, Tex.). Exemplarydifunctional epoxies which may be used in the resin include Bis-Fepoxies, such as GY-281, LY-9703 and GY-285 which are available fromHuntsman Advanced Materials, Inc., Woodlands, Tex.). Bis-A epoxies, suchas GY-6010 (Huntsman Advanced Materials, Inc., The Woodlands, Tex.) andDER 331 (Dow Chemical, Midland, Mich.) are suitable Bisphenol-A typeepoxies and may also be used. An exemplary tetrafunctional epoxy istetraglycidyl diaminodiphenyl methane (MY-721, MY-720 and MY-9512available from Huntsman Advanced Materials, Inc., The Woodlands, Tex.).Other suitable epoxies include phenol novolak type epoxy, cresol novolakepoxy and resorcinol type epoxy. Preferred bis-F epoxies include GY281and GY285 which are available from Huntsman Advanced Materials, Inc.,The Woodlands, Tex.

Exemplary curative agents include dicyandiamide,3,3-diaminodiphenylsulfone (3,3-DDS), amino or glycidyl-silanes such as3-amino propyltriethoxysilane, CuAcAc/Nonylphenol (1/0.1),4,4′-diaminodiphenylsulfone (4,4′-DDS),4,4′-methylenebis(2-isopropyl-6-methylaniline), e.g., Lonzacure M-MIPA(Lonza Corporation, Fair Lawn, N.J.),4,4′-methylenebis(2,6-diisopropylaniline), e.g., Lonzacure M-DIPA (LonzaCorp., Fair Lawn, N.J.). Dicyandiamide and 3,3-DDS are preferredcurative agents. Especially preferred are combinations of 3,3-DDS anddicyandiamide.

Exemplary viscosity control agents include thermoplastic polyetherimidessuch as ULTEM® 1000P which is available from General Electric(Pittsfield, Mass.); micronized polyethersulfone such as 5003P, which isavailable from Sumitomo Chemical Co., Ltd. (Osaka, Japan); HRI-1, whichis available from Hexcel Corp. (Dublin, Calif.); and polyimide MATRIMID®9725, which is available from Huntsman Advanced Materials, Inc. (TheWoodlands, Tex.). ULTEM® 1000P and micronized PES are preferred.Micronized PES is especially preferred. The amount and type of viscositycontrol agent which is added to the epoxy resin mixture may be variedprovided that the minimum viscosity of the final resin mixture ismaintained between 150 and 1500 poise when said viscosity is measured byRheometric Dynamic Analysis (Rheometrics RDA2) at settings of 2° C./mni,10 rads/sec and 0.8-1.0 mm gap. As previously mentioned, mixtures withminimum viscosities of between 300 to 1200 poise are preferred. Theviscosity of the prepreg resin prior to addition of the fillet formingparticles should be between about 50 poise and 2000 poise at roomtemperature. The preferred viscosity range is 100 poise to 1500 poise atroom temperature.

Densified polyethersulfone (PES) and densified polyetherimide particlesmay be used as suitable fillet forming particles. Densified PESparticles are preferred. The densified polyethersulfone (PES) particlesare preferably made in accordance with the teachings of U.S. Pat. No.4,945,154, the contents of which is hereby incorporated by reference.The average particle size of the PES particles range from 1 to 150microns. Average particle sizes of 1 to 50 microns are preferred andaverage particle sizes of 10 to 25 microns are particularly preferred.The microspheres are generally spherical in shape and are classified bypassing the densified microsphere powder through a micron sieve. It ispreferred that the glass transition temperature (Tg) for the particlesbe above 200° C.

The PES which is used may be “micronized”, if desired. Micronized PESrefers to PES particles that have a rough surface configuration that isproduced by grinding the particles or using other abrasive techniques ofmanufacture that are known in the art. Micronized PES particles may alsobe made by using spraying and drying procedures that are also known inthe art. Micronized PES particles are preferably less than 120 micronsin size. Especially preferred are particles under 50 microns in sizewith a range of 10 to 25 microns being particularly preferred.

The prepreg resin is made by first mixing the epoxy components togetherand then slowly adding the polyetherimide or micronized PES viscositycontrol agents. The resulting mixture is heated to around 130° C. andmixed for a sufficient time to dissolve the polyetherimide/PESparticles. Once the polyetherimide/PES is dissolved, the mixture iscooled to around 75° C. The aromatic amine curing agent and the filletforming densified PES particles are then added to the mixture. The resinshould be kept at temperatures below about 70° C.-75° C. while thecurative agent and densified PES particles are being mixed into theresin. The final resin has a minimum viscosity of between 150 to 1500poise when said viscosity is measured by Rheometric Dynamic Analysis(Rheometrics RDA2) at settings of 2° C./min, 10 rads/sec and 0.8-1.0 mmgap. The preferred viscosity range is 300 to 1200 poise.

The finished resin is applied to the desired fabric to form the prepreg.The resin content of the prepreg may be varied depending upon a numberof different parameters in order to achieve desired mechanical andstructural properties for the sandwich panel. It is preferred that theprepreg have a resin content of 35-45 weight percent.

The prepreg is bonded to the edge-coated honeycomb core using vacuumand/or pressure and heated to cure the prepreg and polyamide and/orrubber-containing adhesive to form cured face sheets that are securelybonded to the honeycomb. The amount of vacuum, pressure and heatrequired to cure and bond the prepreg to the edge-coated honeycomb maybe varied depending upon the particular matrix resin formulation, theparticular polyamide and/or rubber-containing adhesive formulation, theamount of resin matrix in the prepreg and the amount of adhesive on thehoneycomb edge. In general, sufficient pressure must be applied to theprepreg to ensure that good contact is made between the honeycomb andprepreg face sheet to provide adequate bonding. When fillet-formingparticles are present in the matrix resin, the temperature and othercuring conditions are also selected such that the densified PESparticles are substantially dissolved during the curing/bonding process.

In accordance with the present invention, a polyamide and/orrubber-containing adhesive is applied to the edges of the honeycombprior to bonding with the composite face sheet. In one embodiment, theadhesive includes polyamide (nylon) as a major ingredient. These typesof adhesives are referred to herein as “nylon adhesives” or “polyamideadhesives” and include any of the known adhesive compounds that includenylon as a major component (at least 40 weight percent and preferably atleast 60 weight percent). Polyamide adhesives are available commerciallyand typically include polyamide in amounts ranging from 60 to 100 weightpercent. Within a polyamide adhesive, other components such as epoxiesmay also be blended, for example, in a concentration of 40-60 weightpercent of the total polyamide adhesive. Exemplary polyamide types thatmay be used include nylon 66, nylon 612, nylon 6. Exemplary polyamidetypes that may be used include nylon 66, nylon 612, nylon 6. Preferredpolyamide adhesives contain transparent thermoplastic polyamides thathave glass transition temperatures of from about 110° C. to 180° C.However, polyamide adhesives having glass transition temperatures in therange of 30° C. to 180° C. may be used, if desired. Polyamide adhesivesthat contain only aliphatic blocks, such as ELVAMIDE, have glasstransition temperatures that are at the lower end of the acceptablerange (i.e. 30° C. to 60° C.). Polyamide adhesives that additionallycontain cycloaliphatic blocks and aromatic blocks, such as GRILAMID,have glass transition temperatures that are at the higher end of theacceptable range (i.e. 100° C. to 160° C.). Only those polyamideadhesives that contain aromatic blocks, such as GRILAMID TR55, haveglass transition temperatures above about 160° C. Polyamide adhesivesthat contain aromatic blocks are particularly preferred for use in hightemperature and high humidity environments. The Tg of the polyamide canbe higher than 160° C. if the percentage of aromatic blocks in TR55 isincreased.

Polyamide adhesives are available commercially from E.I. DuPont deNemours & Company (Wilmington, Del.) under the trade names NYSOL(previously ZYTEL) and ELVAMIDE. NYSOL and ELVAMIDE are preferredpolyamide (nylon) adhesives for use as edge coatings. Other exemplarycommercially available polyamide adhesives are ORGASOL, which isavailable from Atofma Chemicals, Inc. (Philadelphia, Pa.) and GRILAMIDTR55 which is available from EMS Chemie (Sumter, S.C.). Exemplarypreferred polyamide adhesives include ELVAMIDE 8061, 8066 and 8023R; andNYSOL 1020.

The commercially available polyamide adhesives described above aretypically supplied as a solid. The solid polyamide adhesive is dissolvedin a suitable solvent, such as a blend of water and alcohol, to form asolution that is applied to the honeycomb edge. The solvent isevaporated to leave a polyamide coating on the honeycomb edge. Ethanol,benzyl alcohol and combinations of ethanol and benzyl alcohol arepreferred solvents. The amount of solvent that is used to dissolve thepolyamide can be varied to provide a solution having the desiredviscosity for application to the honeycomb edge. In general, thesolutions of polyamide adhesive will include from 10 to 50 weightpercent polyamide.

ELVAMIDE 8061, 8066 and 8023R are exemplary adhesives. The ELVAMIDEadhesives are semi-transparent and contain only aliphatic blocks. Themelting points (ASTM D3418) of ELVAMIDE 8061, 8066 and 8023R are 156°C., 115° C. and 154° C., respectively. The relative viscosities are70-100 for ELVALMIDE 8061, 21-29 for ELVAMIDE 8066 AND 24-36 forELVAMIDE 8023R. The Shore Hardness (ASTMD2240) is 75 for ELVAMIDE 8061,72 for ELVAMIDE 8066 and 70 for ELVAMIDE 8023R. The tensile strength(ASTM D638) is 7,500 psi for ELVAMIDE 8061, 5,700 for ELVAMIDE 8066 and7,400 psi for ELVAMIDE 8023R. The elongation at break (ASTM D638) at 23°C. is 320% for ELVAMIDE 8061, 370% for ELVAMIDE 8066 and 370% forELVAMIDE 8023R.

GRILAMID TR55 and TR90 are transparent thermoplastic polyamides based onaliphatic, cycloaliphatic and aromatic blocks. TR90 contains aliphaticand cycloaliphatic blocks. TR55 contains aliphatic, cycloaliphatic andaromatic blocks. The glass transition temperature (ISO 11357) for TR55is 160° C. and 155° C. for TR90. The tensile E-modulus (ISO 527) forTR55 is 2200 MPa and 1600 Mpa. The tensile strength at yield (ISO 527)for TR55 is 9 MPa and 60 MPa for TR90. The elongation at yield (ISO 527)is 9% for TR55 and 6% for TR90. The tensile strength at break (ISO 527)is 50 MPa for TR55 and 45 MPa for TR90. The elongation at break (ISO527) is greater than 50% for TR55 and greater than 50% for TR90. Theball indentation hardness (ISO 2039-1) for TR55 is 120 MPa and 90 MPafor TR90. The dielectric strength (IEC 60243-1) is 31 kV/mm for TR55 and34 kV/mm for TR90. The comparative tracking index (IEC 60112) for TR55is 600 and 600 for TR90. The heat deflection temperature (ISO 75) at1.80 MPa is 130° C. for TR55 and 115° C. The heat deflection temperatureat 0.45 MPa is 145° C. for TR55 and 135° C. for TR90. GRILAMID TR55 is aparticularly preferred polyamide adhesive for use in accordance with thepresent invention.

The final honeycomb sandwich panel may be designed for use in hightemperature (above 160° F./71 C and above) and/or high humidity (>80percent relative humidity and above) environments. In these situations,it is preferred that polyamide be combined with one or morethermosetting resins to form polyamide adhesives that function well insuch high temperature and/or high humidity conditions. Epoxy, cyanateester, bismalemide and phenolic resins are exemplary of the types ofthermosetting resins that may be combined with polyamide to form hightemperature/high humidity polyamide adhesives in accordance with thepresent invention. Epoxy and phenolic resins are preferred. The resinsmay be used alone or in combination. Exemplary epoxies have beendescribed above. Exemplary phenolics include any of the commerciallyavailable phenolics, such as 23-983 Resin which is available from DurezCorp. (Addison, Tex.), and 23056, available from Durez Corp. (Addison,Tex.). Preferred high temperature/high humidity polyamide adhesivesinclude polyamide in amounts ranging from 40 to 90 weight percentpolyamide and one or more thermosetting resins in amounts ranging from10 to 6 weight percent. Suitable curing agents for the resins need to beincluded as is well know in the art. Coupling agents and otherconventional additives may be included as is also well known in the art.In addition, solvents for the thermosetting resins, such as toluene andxylene, may be used to form resin solutions that are combined withsolutions of polyamide (as described above) to form the hightemperature/high humidity polyamide adhesive solution. The amount ofthermosetting resins in such solutions will typically range from 10 to80 percent by weight.

Rubber in liquid or particulate form may be added to the polyamideadhesive. Polybutadiene rubbers are preferred withacrylonitrile-butadiene copolymer (NBR or Buna-N) being particularlypreferred. Any rubber may be used in combination with polyamide to formthe edge coating adhesive provided that it does not adversely affect theviscosity and adhesive properties of the adhesive. In addition toacrylonitrile-butadiene copolymer rubber, other exemplary rubbersinclude butadiene homopolymer rubber, styrene-butadiene (SBR or Buna-S)rubber, natural (gum) rubber, ethylene-propylene (EPR) rubber,carboxylated nitrile rubber and hydrogenated nitrile rubber. The amountof rubber that is added may be varied depending upon the particularpolyamide adhesive being used. Amounts of rubber on the order of 5 to 20weight percent of the total adhesive are preferred. It is also preferredthat the rubber that is incorporated into the polyamide adhesive is aliquid in order to prevent the viscosity of the polyamide adhesive frombeing too high. Solid rubber may also be used. However, it is preferredthat the solid rubber particles be pre-dissolved in a solvent, such asMEK, and then mixed into the polyamide solution. The addition of rubberimproves the room temperature toughness of the adhesive and reduces theadhesive performance at elevated temperatures. Accordingly, the amountof rubber that is added is selected to provide the desired degree ofincrease in room temperature toughness without reducing the hightemperature performance of the adhesive below desired levels.

In another embodiment of the present invention, rubber forms anessential ingredient of the adhesive with the inclusion of polyamidebeing optional. The rubber-based adhesive preferably contains athermosetting resin in combination with rubber. Such rubber-modifiedthermosetting resins typically contain a thermosetting resin as theprincipal ingredient with rubber being present in amounts that rangefrom 5 weight percent to 50 weight percent of the total adhesive. Any ofthe thermosetting resins mentioned above in connection with thepolyamide-thermosetting adhesive may be used. Such resins includeepoxies, cyanate esters, bismaleimides and phenolics. Epoxies arepreferred. However, it is also preferred that the type of thermosettingresin used in the rubber-modified adhesive be the same or similar to thethermosetting resin that is used as the resin matrix in the face sheetprepreg. For rubber-modified epoxy resins, the amount of rubber ispreferably between about 5 weight percent and 25 weight percent.Particularly preferred rubber amounts are on the order of 15 to 20weight percent.

Any of the various types of rubbers mentioned above can be combined withone or more thermosetting resins to provide an edge-coating adhesive inaccordance with the present invention. Acrylonitrile-butadiene rubber(NBR) is a preferred rubber. NBR, as well as the other types of rubberslisted above are available commercially from a number of sources.Examples include HYCAR CTBN which is available from Noveon SpecialtyChemicals (Cleveland, Ohio) and NIPOL NBR which is available from ZeonChemicals (Louisville, Ky.). NIPOL 1472, HYCAR 1300x13 and combinationsthereof are particularly preferred. HYCAR 1300x13 is a liquid rubberthat has an acrylonitrile content of 26 percent. With respect to thecarboxyl content, the Acid Number is 32 and the Equivalents per HundredRubber is 0.057. The Brookfield Viscosity at 27° C. is 500,000 mPas. Theglass tansition temperature (measured via Differential ScanningCalorimeter) is −39° C. and the functionality of the rubber is 1.8. Themolecular weight is 3150 Mn.

The rubber should have a molecular weight of between 2500 Mn and 60000Mn. The molecular weight(s) of the rubber are chosen to provide anadhesive that has a sufficiently high rubber content (at least 5 weightpercent and preferably 15-20 weight percent) while still having aviscosity that is low enough to allow it to be applied as an edgecoating in accordance with the present invention. The viscosity of therubber solutions and adhesive can be adjusted using solvent to achievedesired viscosity levels. The amount of rubber, solvent and otheringredients are selected to preferably provide a rubber-modifiedadhesive that has a viscosity of between about 1000 and 150000 poise at21° C.

The rubber-modified thermosetting resin may include any number ofadditives that are typically included in such resin systems. Exemplaryadditives include polyamide in amounts that are compatible with therubber in the adhesive. Any of the polyamides described in connectionwith the polyamide adhesive may be used. Other exemplary additivesinclude: flame retardants, such as tetrabromo bisphenol A; colorants,such as antimony trioxide; and anti-foam agents, which may be added intypical amounts for such additives.

The rubber-modified thermosetting resin should include a suitable curingagent, which may or may not be included as part of the thermosettingresin when it is mixed with the rubber component(s). Any of the variouscuring systems and agents may be used as described previously inconnection with the curing of thermosetting resins used in the facesheet prepregs. Tertiary amine catalysts, urea catalysts andcombinations thereof are preferred. Dicyandiamide and/or DDS arepreferred curing agents.

Rubber-modified thermosetting resins are available commercially from anumber of sources. However, these commercial resins typically do notcontain sufficient amounts of rubber to be suitable for use as an edgecoating adhesive in accordance with the present invention. It ispreferred that the rubber-modified thermosetting resin adhesive be madeby combining the thermosetting resin(s) with rubber in a multi-stageprocess. Solid rubber particles (e.g. NIPOL 1472) having a relativelyhigh molecular weight (typically 54000 Mn) are first mixed with asuitable solvent, such as methylethyl ketone (MEK) to form a rubbersolution in which the particles are completely dissolved. This rubbersolution is then added to a second solution that contains a mixture oflower molecular weight rubber (e.g. HYCAR 1300x13) and the thermosettingresin. Catalysts and other additives may be included in the secondsolution. The resulting mixture is heated to temperatures on the orderof 120° C. to 150° C. for a sufficient time (typically 60-120 minutes)to build up molecular weight and form a uniform solution. If desired,additional resin, rubber, additives and/or catalysts and the like can beadded to the two solutions after they have been mixed together andheated (i.e. pre-cooked). This type of staged addition of rubber, wheresolid rubber particles are first dissolved in a solvent and then addedto a liquid rubber/thermosetting resin mixture, is preferred in order toachieve relatively high rubber loading of the adhesive while keeping theviscosity low enough to provide adequate coating of the honeycomb edgeand improved face sheet bonding.

The polyamide and/or rubber-containing adhesive may be applied to thehoneycomb edge using any technique that provides a uniform edge coating.The preferred method is to dip or press the honeycomb edge into a layerof (adhesive solution). The adhesive layer is preferably formed bypouring the polyamide and/or rubber-containing adhesive solution onto aflat surface and spreading the adhesive out using a Gardner's knife ofother suitable blade. The objective is to form a uniformly thick layerthat is from 0.001 inch (0.0254 mm) to 0.050 inch (1.3 mm) thick. Thepreferred range of thickness for the adhesive layer is from 0.003 inch(0.08 mm) to 0.008 inch (0.20 mm). The thickness of the layer may beincreased or decreased outside this range, if desired, depending uponthe thickness of the core, cell size and wall thickness as well as theparticular face sheets being bonded to the honeycomb. The thickness ofthe adhesive solution layer determines the depth (see “D” in FIG. 3) ofthe coating on the honeycomb edge. The preferred depth of the edgecoating for any given honeycomb-face sheet combination may be determinedby routine experimentation involving varying the edge coating depth andmeasuring the resulting bond (peel) strength of the honeycomb-face sheetbond.

The honeycomb edge is coated with adhesive by simply placing thehoneycomb edge onto the adhesive layer and moving the honeycomb downwardto contact the underlying surface. The honeycomb is then pulled awayfrom the underlying surface with adhesive solution remaining only on theportion of the honeycomb walls that was immersed in the adhesive layer.The solvent, if any, is evaporated to leave a coating of polyamideand/or rubber-containing adhesive on the honeycomb edge. Edge coating ofthe honeycomb in this manner is well know and preferred since itprovides a simple and effective method for applying a uniform coating ofadhesive to the edge of the honeycomb. Other methods involving powdercoating, spray, roller or brush application of the adhesive may also beused, if desired, especially for edge coating of relative largehoneycomb core.

The amount of polyamide and/or rubber-containing adhesive that isapplied to the honeycomb edge may be varied provided that the desireddegree of bond strength is obtained without adding too much weight tothe panel structure. The amount of polyamide adhesive that provides theoptimum combination of high bond strength and light weight can also bedetermined by routine experimentation. This is achieved by not onlyvarying the depth (D) of the edge-coating, but also varying theviscosity or tackiness of the polyamide adhesive solution. Also, afterevaporation of the solvent, the core can be dipped again to pick upadditional adhesive. This re-dipping process can be repeated as manytimes as necessary to achieve the desired level of adhesive loading. Thedrying temperature depends on the solvent used in the polyamidesolution. For water-based coatings, the drying time is typically 4minutes at 121° C. The amount of polyamide adhesive applied to thehoneycomb edge is preferably between 0.5 g sq.ft. (5.4 g/sqm) and 10g/sq. ft. (107.6 g/sqm). However, the amount of adhesive that is usedcan be varied widely depending upon the intended use for the honeycombsandwich panel and the desired combination of panel weight and facesheet peel strength.

The bond formed by the polyamide and/or rubber-containing adhesive andface sheet resin matrix is preferably as strong or stronger than thestructural strength of the honeycomb. For example, it is preferred thatthe polyamide adhesive, prepreg face sheet and honeycomb are chosen suchthat (when the face sheet is forcibly pulled from the honeycomb) thebody of the honeycomb (shown at 50 in FIG. 4) separates (see arrows 52)from the face sheet 53 prior to failure of the bond 54 between thehoneycomb and face sheet bond. The use of a polyamide adhesive edgecoating in combination with composite face sheets in accordance with thepresent invention provides sufficiently high bond strength between theface sheet and honeycomb so that it is routinely possible to makesandwich panels where at least a portion of the core suffers structuralfailure prior to bond failure during peel tests. The particularly highbond strengths provided by the present invention allows one to obtainmaximum structural strength from a panel structure where the overallstrength of the panel is limited by the structural strength of the coreand not the bond between the face sheet and core.

The sandwich panels are particularly well suited for use as aircraftfloor panels for use in large aircraft, as shown at 60 in FIG. 5. Thefloor panels present in the aircraft 60 include aisle floor panel 62,under seat floor panel 64; high load floor panel 66; and galley floorpanels 68. All of the floor panels are preferably on the order of 0.30to 0.50 inch thick with 0.40 inch thick panels being particularlypreferred. The weight of the floor panels will vary depending upon typeand thickness. For panels that are 0.40±0.01 inch thick, it is preferredthat the panel weights be equal to or less than: 0.41 lbs/sq. ft. foraisle floor panels; 0.47 lbs/sq. ft. for underseat floor panels; 0.64lbs/sq. ft. for galley; and 0.90 lbs/sq. ft. for high load floor panels.

The long-beam bending of the panels (MIL-STD-401B, 20-inch span, ¼ pointloading) is preferably at least about: 300 lbs for aisle floor panels;310 lbs for under seat floor panels; and 360 lbs for galley and highload floor panels. The long beam deflection at 100 lbs (L-STD-401B,20-inch span, ¼ point loading) is preferably no more than about: 0.38inch for aisle floor panels; 0.035 inch for under seat floor panels; and0.31 inch for galley and high load floor panels. The panel shearstrengths (MIL-STD-401B) are preferably higher than: 500 lbs for aislefloor panels; 800 for under seat floor panels; 1200 for galley and highload floor panels. The drum peel strength (MIL-STD-401B) of the facesheets is preferably at least: 35 in-lbs/3 inch for the aisle floorpanels; and 50 in-lbs/3 inch width for the other three types of floorpanels.

The floor panels can be made from a variety of cores and face sheetsprovided that the polyamide dip adhesive is used and the above physicaland structural parameters are met. The preferred core material for floorpanels are aramid fiber/epoxy resin composites, such as HexWeb®HRH36.The cell size is preferably about ⅛ inch to 3/16 inch. The density ofthe core (wall thickness) is varied to provide the desired weight andstructural limits as set forth above depending upon the type of floorpanel. Relatively low density core (3-10 lb/cu. ft.) is used for aisleand under seat floor panels and medium density (8-12 lb/cu. ft.) is usedfor the galley and high load floor panels.

The face sheets for the floor panels are preferably formed usingprepregs composed of woven carbon fiber tows. There preferably are 1200carbon filaments per tow and the tows are woven to provide a cloth thathas an areal weight of about 315 gsm. The resin content of the prepregmay range from 30 to 50 percent by weight. Resin contents of about 38weight percent are preferred.

Both of the edges of the honeycomb are preferably coated with apolyamide adhesive, such as ELVAMIDE 8066 or GRILAMID TR55, which arecomposed essentially of nylon. The amount of polyamide adhesive that isapplied to each of the honeycomb edges is preferably from 10 gsm to 80gsm. The floor panels are formed following the same procedures set forthabove for forming panels in accordance with the present invention. Infabricating the floor panels, various combinations and amounts ofhoneycomb, prepreg face sheets and dip resin may be used provided thatthe final floor panel meets the weight and structural parameters setforth above for the specified panel type.

The sandwich panels are also particularly well suited for use as panelsthat form non-structural components in the interiors of large aircraft.A few exemplary interior components that can be made using aircraftinterior sandwich panels in accordance with the present invention areshown in FIG. 5. They include overhead storage bins 70, ceiling panels72 and bulkhead panels 74. Other types of panels include lavatorypanels, wardrobe panels, galley panels, partition panels, sidewallpanels and the like.

Since the interior panels are present in the interior of the aircraft,it is preferred that the resin components are resistant to hightemperatures and fire. In addition, when the resins do burn, it ispreferred that they produce low amounts of smoke and other noxiousfumes. Accordingly, the aircraft interior panels of the presentinvention preferably have face sheet that are made from composites thathave a phenolic resin matrix. Any of the phenolic resin compositionsthat are know to be high temperature and flame resistant may be used. Inaddition, the core material is preferably honeycomb made using a resinmatrix that is also a high temperature/flame resistant phenolic resin.

The honeycomb should have a density of between 1.5 to 4.0 pcf and theoverall interior panels should weigh between 0.2 and 0.4 pounds persquare foot (psf). This weight range provides the desired combination ofstrength and light weight that is needed for a panel to be acceptablefor use in the interior of large aircraft. The prepreg face sheet,honeycomb and edge-coating adhesive are chosen such that the interiorpanel meets the psf weight limitation for the interior panel. Preferredexemplary cores are aramid paper cores impregnated with phenolic resin,such as HexWeb HRH36 honeycomb. Preferred honeycomb have ⅛-inch cellsand are from 0.2 to 0.5 inch thick.

It is preferred that each edge of the interior panel honeycomb be coatedwith from 0.5 to 3.0 gsf or 5 to 30 grams per square meter ofedge-coating adhesive. Total edge-adhesive for the panel will thereforerange from 1.0 to 6.0 gsf or 10 to 60 gsm. Preferred adhesives are thosethat contain mainly polyamide. Edge coating adhesives that containrubber and/or resin, as discussed in detail above, may be used providedthat the amount of rubber and/or resin that is present in the panel doesnot cause the generation of undue amounts of smoke when burned.

Examples of practice are as follows:

Comparative Example 1

A prepreg matrix resin was prepared having the following formulation:

23 weight percent MY-0510 (N,N-Diglycidyl-4-glycidyloxyaniline);

25 weight percent GY281 (bis-F epoxy);

19 weight percent 3,3-Diaminodiphenylsulfone (3,3-DDS);

7 weight percent ULTEM® 1000P (polyetherimide); and

26 weight percent densified PES.

The densified PES was made from PES 5003P which is available fromSumitomo Chemical Co. Ltd. (Osaka, Japan). The PES was densified inaccordance with U.S. Pat. No. 4,945,154. MY0510 and GY281 were firstmixed in a mixing vessel, heated to 70° C. for approximately 10 minutes.The ULTEM® 100P particles were then added and the resulting mixtureheated to 130° C. with mixing for approximately 75 minutes to fullydissolve the ULTEM® 100P particles. The mixture was then cooled to 75°C. and the 3,3-DDS was mixed in for about 15 minutes. Then, thedensified PES was slowly added and mixed in for approximately 10 minutesto provide the final resin mixture. The viscosity of the homogeneousresin was measured over the entire curing temperature range (i.e., 20°C. to 177° C.) using Rheometric Dynamic Analysis as previouslydescribed. The resin had a minimum viscosity of 900 poise.

Face sheets were prepared by first forming a prepreg of 193 gsm plainweave (PW) fabric woven from 3000 filament (3K) carbon fiber towsimpregnated with 138 grams of resin per square meter. The prepreg wasformed as follows:

The resin was coated on release paper by reverse roller at about 175° F.(79° C.) to form a film containing 69 g/m². Two resin films wereimpregnated into the carbon fiber (in the form of the PW woven fabric)with an areal weight of 193 g/m².

The prepreg was applied to HRH® 10 core having ⅛ inch (0.31 cm) cellsand being ½ inch (1.27 cm) thick under vacuum at 22 inches (56 cm) Hgand cured for 2 hours at 177° C. with a pressure of 45 psi, venting at20 psi and ramp cooling at a rate of 2° C. per minute. The resultingspecimens were subjected to peel test according to ASTM D 1781. The facesheets all had peel strengths between 29 and 33 in-lb/3 in width.

Comparative Example 2

A prepreg matrix resin was prepared in the same manner as ComparativeExample 1 except that the ingredients used to make the resin were asfollows:

21 parts by weight MY-0510;

21 parts by weight AcroCy® L-10;

21 parts by weight GY281;

9 parts by weight ULTEM® 1000P;

1.5 parts by weight CuAcAc/Nonylphenol (1/0.1); and

26.5 parts by weight densified PES.

The minimum viscosity of the homogeneous resin mixture was found to beabout 500 poise. The viscosity of the final resin mixture was measuredas set forth in Example 1. The final resin mixture was used to form aprepreg and applied to HRH® 10 core in the same manner as Example 1. Thepeel strength of the resulting face sheet was 26 in-lb/3 in width.

Comparative Example 3

A prepreg matrix resin was prepared having the following formulation:

27.0 weight percent MY-0510 (N,N-Diglycidyl-4-glycidyloxyaniline);

24.9 weight percent GY285 (bis-F epoxy);

15.8 weight percent 3,3′-Diaminodiphenylsulfone;

1.3 weight percent Dicyandiamide;

13.5 weight percent micronized Polyethersulfone (PES); and

17.5 weight percent densified Polyethersulfone (PES).

Resin formulations may also be made wherein the amounts of MY-510, GY281and 3,3-DDS are varied by up to ±15%. Also, the amounts of both types ofPES may be varied by as much as ±40%. The amount of dicyandiamide may bevaried by up to ±50%.

The densified PES was the same as used in Comparative Examples 1 and 2.Average particle size was 10-25 microns with no more than 13 weightpercent smaller than 5 microns and no more than 4 weight percent greaterthan 40 microns. 24.9 parts by weight of GY285 and 6.0 parts by weightof MY0510 were mixed in a resin kettle and heated, with stirring, to 65°C. Once this temperature is attained, 13.5 parts by weight micronizedPES 5003P is added to the resin kettle. The mixture is then heated to128±2° C. and held at this temperature for 75 minutes. At the end of 75minutes, heating is removed and 21 parts by weight of MY0510 are addedto the kettle. Stirring is continued as the mixture cools to 65° C. 15.8parts of 3,3-DDS is added and mixed for 15 minutes. 1.3 parts ofdicyandiamide is then added and the mixture stirred for 5 minutes at 65°C. Finally, 17.5 parts of densified PES is added and stirred in for 10minutes. The minimum viscosity of the resin was measured as set forth inExample 1 and found to be about 370 poise. Panels were prepared by firstforming a prepreg of 193 gsm 3K PW carbon fabric containing 70 grams ofresin per square meter. The prepreg was formed as follows:

The resin was coated on release paper by reverse-roll coater at about165° F. (74° C.) to form a film containing 70 g/m². The resin film wasimpregnated into a carbon fiber fabric having an areal weight of 193g/m². The prepreg was then applied to HRH® 10 core and cured in the samemanner as Comparative Example 0.1. The peel strength was around 32in-lb/3 in width on 3 pound core and around 31 in-lb/3 in width on 8pound core.

Comparative Example 4

Sandwich panels were prepared in the same manner as Comparative Example3 except that HRH®36, 2.5 pcf made with KEVLAR® was used as the coreinstead of HRH®10. The peel strength was found to be between 12 and 18in-lb/3 in width.

Comparative Example 5

Sandwich panels were prepared in the same manner as Comparative Example4 except that the HRH®36 core was edge coated with various coatings thatdid not include a polyamide adhesive. One of the materials was A-22,which is a commercially available anionic surfactant that is availablefrom Cytec Industries, Inc. (West Paterson, N.J.). As described inUS00/5575882A, a significant increase in honeycomb modulus was observedafter immersing the core into a 2% by weight solution of A-22 anionicsurfactant. A second edge-coating material was 23056 which is a typicalphenolic resin for non-metallic core such as HRH®10 core. 23056 isavailable commercially from Durez Corp. (Addison, Tex.). A thirdmaterial was Phenolic coating 94-917, which is a modified phenolic thatis used as a coating for cans. Phenolic coating 94-917 is availablecommercially from Durez Corp. (Addison, Tex.). The honeycomb edges wherecoated with about 6.3 g/sq. ft. of each coating. All of the peelstrengths for the resulting panels were below 11 in-lbs/3 in width.

Example 1

Sandwich panels are prepared in the same manner as Comparative Examples1 and 2, except that the edges of the honeycomb are coated with anaqueous solution of NYSOL 1020 (also know as ZYTEL FE310018) polyamideadhesive (40 weight percent polyamide) so that depth “D” of the adhesiveis between 0.005 inch (0.13 mm) and 0.008 inch (0.20 mm). The adhesive(coating) is applied to the honeycomb edges as follows: 1) a flat andlevel glass table top is cleaned with a degreasing solvent to remove alldebris and contaminants; 2) A Gardner's knife is set on the tabletop andthe gap setting at both ends of the knife are adjusted to provide thedesired film thickness; 3) The polyamide solution is poured onto thetable top parallel to the Gardner's knife and the knife is then draggedacross the adhesive with equal pressure on both ends of the knife toform a thin film that is slightly larger than the area of the core; 4)the resulting film is visually inspected to insure that a uniform filmof constant thickness has been formed; 5) the core is placed on top ofthe film and manually pressed downward with a consistent pressure toinsure uniform application of the adhesive film to the core; 6) the coreis then lifted from the table top and dried in an electric oven at 250°F. (121° C.) for 4 minutes; 7) the core is weighed to determine how muchnylon adhesive has been retained on the core edge before and aftercoating; and 8) the process is repeated, if necessary to obtain thedesired polyamide coating loading which in this example is between 3 and10 grams per square foot (g/sq. ft.). The panels prepared in thisexample will have peel strengths on the order of 90 in-lbs/3 in width.The adhesive bond between the core edges and the skins or face sheetsare sufficiently strong that the honeycomb wall, and not the core-skinbond, will structurally fail over at least a part of the honeycomb edgeduring the peel test.

Example 2

Sandwich panels were made in the same manner as Example 1 except thatthe prepreg face sheets or skins were the same as those used inCOMPARATIVE EXAMPLE 3. The HRH®10 (8 pcf) core was edge coated with theNYSOL 1020 nylon adhesive solution to provide an adhesive loading of 4.3g/sq. ft. and the depth D of the coating on the core edge was about0.005 inch (0.13 mm). The peel strength of this panel was 91 in-lbs/3 inwidth with structural failure occurring in the honeycomb wall over asubstantial portion of the honeycomb and not at the core-skin bond.

Example 3

Sandwich panels were made in the same manner as Example 2 except thatHRH®36 (2.5 pcf) core was substituted in place of HRH® 10 core. Theedges of six different cores were coated with 1.1, 2.3, 3.4, 3.8, 4.0and 6.2 g/sq. ft. of polyamide adhesive (i.e., NYSOL 1020, 40% solidssolution in water, as per Example 1). The resulting panels had peelstrengths of 26, 30, 59, 71, 71 and 76 in-lbs/3 in width, respectively.During the peel tests, the face sheets separated from the core at thebond between the face sheet and the core. Based on this example, it ispreferred for this particular type of sandwich panel configuration thatthe edge of the honeycomb be coated with at least 3.0 g/sq. ft. ofpolyamide adhesive.

Example 4

Sandwich panels were made in the same manner as Example 3 except thatELVAMIDE 8023R was used in place of NYSOL 1020 in the solution that wasapplied to the honeycomb. The solution was composed of: 64 weightpercent ELEVAMDE 8023R; 16 weight percent ethanol; 10 weight percentbenzyl alcohol; and 9 weight percent water. The edges of three differentcores were coated with sufficient solution to provide loadings of 1.8,2.9 and 3.7 gm/sq. ft. of nylon adhesive. The resulting panels had peelstrengths of 37, 39 and 42 in-lbs/3 in width, respectively. During thepeel tests, the face sheets separated from the core at the bond betweenthe face sheet and the core. Based on this example, it is preferred forthis particular type of sandwich panel configuration that the edge ofthe honeycomb be coated with at least 1.0 gm/sq. ft. of nylon adhesivethis coating.

Example 5

Sandwich panels were made in the same manner as Example 3 except thatthe prepreg face sheets were made using F593 Resin which is availablefrom Hexcel Corporation. The panels had peel strengths on the order of63 in-lbs/3 in width when the core edges contained NYSOL 1020 polyamidecoating weights of about 5.5 g/sq. ft. When the face sheets wereattached to the core without the polyamide edge coating, the peelstrength was only about 5 in-lbs/3 in width.

Example 6

Sandwich panels are made in the same manner as Example 3 except that ahigh temperature/high humidity polyamide adhesive is used in place ofNYSOL 1020. The polyamide adhesive is preferably composed of ELVAMIDE8023R in combination with epoxy and phenolic resins along with asuitable amount of a curing agent and/or catalyst for the resins (e.g.4,4′-diaminodiphenylsulfone (4-4-DDS)) and a coupling agent, such asZ-6040 which is available from Dow-Corning (Midland, Mich.). In atypical formulation, a 25 to 30 weight percent solution of ELVAMIDE8023R (ethanol:benzyl alcohol-1:2 by weight) is first prepared. Thispolyamide solution is then combined with from 15 to 25 weight percent ofa novolak epoxy solution (e.g. 45 to 50 weight percent ECN 1299 epoxy intoluene/xylene) and 5 to 15 weight percent phenolic resin (e.g. 23-983Resin which is available from Durez Corp. (Addison, Tex.). 0.5 to 2weight percent of a curing agent (e.g. 4-4-DDS); 0.1 to 0.5 weightpercent of a phenylurea catalyst (e.g. DIURON available from E.I. DuPontde Nemours & Company (Wilmington, Del.); and 0.1 to 0.5 weight percentof a coupling agent (e.g. Z-6040) are also included. Other additives,such as AGERITE D Resin (RT Vanderbilt of Norwalk, Conn.) may be addedin small quantities (e.g. less than 1 weight percent).

The peel strengths of the sandwich panels made with the formulationabove with 2 to 4 g/sq. ft. polyamide adhesive loading, when measured atroom temperature under dry conditions, will generally be about 32in-lbs/3 in width. The peel strengths of the same sandwich panels willgenerally only drop to about 25 in-lbs/3 in width when measured underwet conditions (e.g., 95% relative humidity or higher) at an elevatedtemperature of 71° C. As nylon appears to be more susceptible to waterpick up than epoxy, with polyamide adhesives comprising pure nylon,typical readings for peel strength are 37 to 42 in-lbs/3 in width atroom temperature (dry), and about 20 to 34 in-lbs/3 in width atconditions of elevated hot/wet.

Example 7

Sandwich panels were made in the same manner as Example 3 except thatELVAMIDE 8066 was used in place of NYSOL 1020 as the polyamide adhesive.The solution that was applied to the honeycomb was composed of: 16weight percent ELVAMIDE 8066; 38 weight percent ethanol; 23 weightpercent benzyl alcohol and 23 weight percent water. The edges of twodifferent cores were coated with sufficient solution to provide edgeloadings of 1.3 and 3.9 gm/sq. ft. of polyamide adhesive. The resultingpanels had peel strengths of 19.5 and 56.3 in-lbs/3 in width,respectively. Panels made with cores that were coated with 4.0 gm/sq.ft. of ELVAMIDE 8066 adhesive had FWT's of about 450 psi at 93° C. andabout 270 psi at 135° C.

Example 8

Sandwich panels were made in the same manner as Example 3 except thatGRILAMID TR55 was used in place of NYSOL 1020 as the polyamide adhesive.The edges of two different cores were coated with 2.3 and 4.8 gm/sq. ft.of polyamide adhesive. The resulting panels had peel strengths at roomtemperature of 22.8 and 26.0 in-lbs/3 in width, respectively. The peelstrengths of the two panels increased to 24.6 and 28.6 in-lbs/3 inwidth, respectively, when the tests were conducted at 71° C. under wetconditions. Three additional panels were made with edge coatings of 4.1,5.2 and 5.8 gm/sq. ft. of GRILAMID TR55. The panels had peel strengthsof 24, 34 and 39 in-lbs/3 in width, respectively, at room temperature.The peel strengths of the panel increased to 25, 38 and 50 in-lbs/3 inwidth, respectively, at 93° C. The FWT for these three panels at 93° C.was 767 psi, 808 psi and 891 psi, respectively. At 135° C., the FWT forthe same panels decreased to 579 psi, 543 psi and 597 psi. Peelstrengths of the two panels (2.3 and 4.8 gm/sq. ft.) increased to 24.6and 28.6 in-lbs/3 in width, respectively, when the tests were conductedat 71° C. under wet conditions. As noted previously, GRILAMID TR55 isbased on aromatic blocks and has a relatively high glass transitiontemperature (Tg) when compared to other polyamide adhesives that containonly aliphatic and cycloaliphatic blocks. Accordingly, polymeradhesives, such as GRILAMID TR55, which have relatively high Tg's, arebetter suited for use at elevated temperatures and/or in a wetenvironment.

Example 9 Aircraft Aisle Floor Panel

Aircraft aisle floor panels were made in the same manner as the panelsdescribed in Example 8. The panel cores were HRH®36 (7.0 pcf). The twoface sheets were formed from prepreg composed of 1 ply of 12,000filament (12K) intermediate modulus IM7 or IM7C type carbon fibers(HEXCEL CORP., Dublin, Calif.) which were woven to provide a 5 harnesssatin weave. The fibers were impregnated with resin to provide a 315 gsmprepreg which was composed of about 38 weight percent resin. The resinhad the following formulation:

47 weight percent DER331 or GY2600 (reaction products of epichlorohydrinand Bis Phenol A)

9 weight percent Polyvinyl Formal (PVF)

16 weight percent Epon 1009F (bisphenol A/epichlorohydrin resin);

9 weight percent tetra bromo bis phenol A (TBBPA);

8 weight percent DER332, Diglycidal Ether of bis Phenol A

3 weight percent antimony trioxide

5 weight percent dicyanamide

3 weight percent diuron

The resin was prepared as follows: MEK solvent and DER331 or GY2600 wereadded to a mixing vessel and stirred. PVF was added while mixing. TheMix was heated to 180° F. while stirring in a reflux process. After 3½hours the mix was cooled to 150° F. to check dissolution, and thereaftercooled to 130° F. while continuing to stir. Epon 1009F was added andstirred until dissolved. TBBBPA, DER332, and antimony trioxide wereadded while stirring. Dicyanamide and Diuron were then added whilestirring.

Face sheets were prepared by first forming prepreg of 315 gsm 12K IM7C5HS fabric containing 38% of resin. The prepreg was formed as follows:

The resin solution was placed in a dip pan and the fabric was passedthrough the resin solution, then the MEK was driven off in a verticaltower oven. The prepreg was rolled up with an interleaf sheet of polyfilm and stored in a freezer until needed.

The prepreg was applied to HRH® 36 honeycomb core having ⅛ inch (0.31cm) cells and being 0.375 inch (9.5 mm) thick in a heated flat platenpress and cured for 1 hour at 130° C. with a pressure of 50 psi. Aftercure, the press was cooled under pressure to 40° C. and then the panelswere removed. Specimens cut from the panels were subjected to peel testaccording to ASTM D 1781. The face sheets all had peel strengths about37 in-lb/3 in width.

The edges of the honeycomb were coated with 45 gsm Elvamide 8066 priorto application of the face sheets to the core and curing.

These aisle floor panels had an overall thickness of 0.40±0.01 inch andweighed 0.47±0.02 lb/sq. ft. The thickness of the face sheets (skins)was 0.012±0.001 inch. The long beam bending for the panels was 331±20lbs and the long beam deflections at 100 lbs were 0.35±0.005 inch. Theshear strength of the panels was 862±25 and the drum peel strengths were37±6 in-lbs/3″ width.

Example 10 Aircraft Under Seat Floor Panel

Aircraft under seat floor panels were made in the same manner as theaircraft aisle panels described in Example 9. The panel cores wereHRH®36 (5.0 pcf). The two face sheets were formed from prepreg composedof 1 ply of 12 K IM7 or IM7C type graphite fibers which were woven toprovide a 5 harness satin weave. The fibers were impregnated with thesame resin used in Example 9 to provide a 315 gsm prepreg which wascomposed of about 38 weight percent resin. The edges of the core werecoated with 45 gsm Elvamid 8066 prior to application of the face sheetsto the core and curing.

The under seat floor panels had an overall thickness of 0.40±0.01 inchand weighed 0.40±0.02 lb/sq. ft. The thickness of the face sheets(skins) was 0.012±0.001 inch. The long beam bending for the panels was311±20 lbs and the long beam deflections at 100 lbs were 0.36±0.005inch. The shear strength of the panels was 602±150 and the drum peelstrengths were 57±20 in-lbs/3″ width.

Example 11 Aircraft Galley Floor Panel

Aircraft galley floor panels were made in the same manner as the panelsdescribed in Example 9. The panel cores were HRH®36 (10.5 pcf). The twoface sheets were formed from prepreg composed of 1 ply of 12 K IM7 orIM7C type graphite fibers which were woven to provide a 5 harness satinweave. The fibers were impregnated with the same resin used in Example 9to provide a 400 gsm prepreg which was composed of about 38 weightpercent resin. The edges of the core were coated with 45 gsm Elvamid8066 prior to application of the face sheets to the core and curing.

The galley floor panels had an overall thickness of 0.40±0.01 inch andweighed 0.63±0.02 lb/sq. ft. The thickness of the face sheets (skins)was 0.015±0.001 inch. The long beam bending for the panels was 394±20lbs and the long beam deflections at 100 lbs were 0.29±0.005 inch. Theshear strength of the panels was 1304±50 and the drum peel strengthswere 58±20 in-lbs/3″ width.

Example 12 Aircraft High Load Floor Panel

Aircraft high load floor panels were made in the same manner as thepanels described in Example 9. The panel cores were HRH®36 (12.5 pcf).The two face sheets were formed from prepreg composed of 2 plies of 12 KIM7 or IM7C type graphite fibers which were woven to provide a 5 harnesssatin weave. The fibers were impregnated with the same resin used inExample 9 to provide a 315 gsm prepreg which was composed of about 38weight percent resin. The edges of the core were coated with 45 gsmElvamid 8066 prior to application of the face sheets to the core andcuring.

The high load floor panels had an overall thickness of 0.40±0.01 inchand weighed 0.86±0.02 lb/sq. ft. The thickness of the face sheets(skins) was 0.024±0.002 inch. The long beam bending for the panels was699±20 lbs and the long beam deflections at 100 lbs were 0.17±0.005inch. The shear strength of the panels was 1600±50 and the drum peelstrengths were 60±10 in-lbs/3″ width.

Example 13

Panels were made in accordance with Example 8 except that GRILAMID TR90was substituted for GRILAMID TR55. The peel strength for panels havingcore edges coated with 5.2 gm/sq. ft. GRILAMID TR90 was about 23in-lbs/3 in width at room temperature and 27 in-lbs/3 in width at 93° C.The FWT at 93° C. was 639 psi. At 135° C. the FWT was 519 psi.

Example 14

Panels were made in accordance with Example 8 except that mixtures ofELVAMIDE 8066 and GRILAMIDE TR55 were substituted for GRILAMID TR55.Three different mixtures of the two polyamides (Type A, Type B and TypeC) were prepared. In Type A, the polyamide mixture included 65 weightpercent ELVAMIDE 8066 and 35 weight percent GRILAMID TR55. In Type B,the polyamide mixture included 50 weight percent of each polyamideadhesive. In Type C, the polyamide mixture included 25 weight percentELVAMIDE 8066 and 75 weight percent GRILAMID TR55. Solutions containingthe Type A and Type B polyamide mixtures were coated onto the honeycombedge to provide adhesive weights of 4.0 gins/sq. ft. The peel strengthswere 34 and 42 in-lbs/3 in width, respectively, at room temperature. TheFWT at 93° C. was 459 psi for the Type A edge coating and 628 psi forType B. At 135° C., the FWT dropped to 292 psi for Type A and 344 psifor Type B. The Type B polyamide adhesive combination was also coated ata rate of 3.9 gms/sq. ft. As expected, the peel strengths were lowerthan the panels coated at a 4.0 gms/sq. ft. rate. However, the FWT at93° C. increased to about 660 psi. At 135° C., the FWT for the slightlylower loaded panel also increased to 391 psi.

The Type C combination was edge-coated onto the cores at a rate of 4.3gms/sq. ft. The peel strengths were about 30 in-lbs/3 in width at bothroom temperature and 93° C. The FWT at 93° C. was 659 psi. At 135° C.,the FWT dropped to 370 psi.

Example 15

Panels were made in the same manner as Example 14, except that a smallamount of ECN 1299 epoxy was added to the ELVAMIDE 8066/GRILAMID TR55polyamide adhesive. Three different combinations of adhesive solutions(Type D, Type E and Type F) were prepared. The Type D adhesive solutioncontained 6.9 weight percent ELVAMIDE 8066, 6.9 weight percent GRILAMIDTR55, 1.2 weight percent ECN 1299, 83.1 weight percent benzyl alcoholand 1.8 weight percent acetone. The Type E adhesive solution contained2.9 weight percent ELVAMIDE 8066, 8.2 weight percent GRILAMID TR55, 1.0weight percent ECN 1299, 86.4 weight percent benzyl alcohol and 1.5weight percent acetone. The Type F adhesive contained 2.9 weight percentELVAMIDE 8066, 8.7 weight percent GRILAMID TR55, 0.61 weight percent ECN1299, 86.9 weight percent benzyl alcohol and 0.9 weight percent acetone.The peel strengths for all three of the adhesive types ranged from 28 to31 in-lbs/3 in width at both room temperature and 93° C. for coatingweights of about 4.2 g/sq. ft. The FWT for the Type D adhesive was 527psi at 93° C. and 365 psi at 135° C. The FWT for the Type E adhesive was700 psi at 93° C. and decreased to 456 psi at 135° C. The FWT for theType F adhesive was 696 psi at 93° C. and 442 psi at 135° C.

Comparative Example 6

A sandwich panel was made in the same manner as Comparative Example 4except that the face sheet prepreg was made using an epoxy resin matrixthat is available from Hexcel Corporation under the tradename F155. Thehoneycomb was not edge coated and a separate structural adhesive was notapplied to the prepreg. The peel strength for the panel was 10 in-lbs/3in width at room temperature. The FWT was 579 psi at room temperatureand 501 psi at 71° C.

Example 16

Sandwich panels were made in the same manner as Comparative Example 6except that the HRH-36 core was edge coated with a rubber-modified epoxyadhesive in accordance with the present invention. The adhesives wereprepared as follows. A rubber solution (Solution 1) was prepared bymixing 85 parts by weight MEK with 15 parts by weight NIPOL 1472. Themixture was kept at room temperature for 3 to 4 hours until the NIPOL1472 rubber particles had completely dissolved. A second solution wasprepared by mixing 302 grams of Solution 1 with 172 grams bisphenol A, 3grams of a tertiary amine catalyst, 136 grams of DER 331 epoxy resin,317 grams of GY 282 epoxy resin and 68 grams of HYCAR 1300x13 liquidrubber. All of these materials were mixed together and heated to 140° C.to 150° C. for 90 minutes to form Solution 2.

45 grams of Solution 2 was then mixed with 65 grams MEK, 32 grams ofGY2600 epoxy, 32 grams of pulverized dicyandiamide and 15 grams of aurea catalyst to provide a Solution 3. A first adhesive was prepared bymixing 300 grams of Solution 3 with 150 grams of a rubber solution thatcontained 20 weight percent NIPOL 1472 rubber particles dissolved inMEK. The total rubber content for this rubber-modified epoxy adhesive(RBE 1) was 16.6 weight percent. A second adhesive was prepared bymixing 265 grams of Solution 3 with 185 grams of the rubber solutionthat contained 20 weight percent NIPOL 1472. The total rubber contentfor this adhesive (RBE 2) was 20.4 weight percent.

The HRH-36 honeycomb was coated with 5.2 gms/sq. ft. of RBE 1 or 5.4gms/sq. ft. of RBE 2 prior to bonding of the face sheet to the core. Atroom temperature, the peel strengths for the panels were 41 and 52in-lbs/3 in width for RBE 1 edge coated cores and RBE 2 edge coatedcores, respectively. Corresponding peel strengths at 71° C. were 44 and50 in-lbs/3 in width, respectively. The FWT for RBE 1 coated cores atroom temperature and 71° C. were 708 psi (due to core failure) and 530psi, respectively. The FWT for RBE 2 coated cores at room temperatureand 71° C. were 693 psi (due to core failure) and 455 psi, respectively.

Example 17

A sandwich panel was made in the same manner as Example 12 except thatthe HRH-36 core was coated with 4.0 gms/sq. ft. of the polyamide edgecoating of Example 4, except that ELVAMIDE 8066 was substituted forELVAMIDE 8023R. The peel strength was 47 in-lbs/3 in width at roomtemperature. The FWT at room temperature was 665 psi (due to corefailure) and the FWT at 71° C. dropped to 396.

Example 18 Aircraft Interior Panel

Sandwich panels were made for use in the interior of a large commercialaircraft. The aircraft interior panels were made in the same basicmanner as the panels described in the preceding examples. The honeycombwas HexWeb®HRH-36 that had ⅛-inch cells. The honeycomb had thicknessesranging from 0.375 inch to 0.5 inch. The density for all of thehoneycomb cores was 2.5 pcf. The honeycomb dip resin was a phenolicresin.

The two face sheets where made using CYCOM®2290 which, as previouslymentioned, is a standard 7781 phenolic/fiberglass prepreg. ELVAMIDE 8066was used as the edge coating in the same manner as Example 7. Threedifferent panels were made with the amount of adhesive being 0.5, 1.5and 2.0 gsm for each edge of the honeycomb (1.0, 3.0 and 4.0 gsm fortotal panel).

The panels were all about 48 inch wide and from 45 to 50 inches long.The 0.375-inch thick panels weighed from about 4.3 to 4.5 lbs (arealweights of from 0.284 to 0.294 psf). The 0.5-inch thick panels weighedfrom 4.8 to 5.2 lbs (areal weight of from 0.313 to 0.318 psf). The peelstrengths ranged from 10 to 30 in-lbs/3 in width at room temperature asthe amount of edge adhesive increased from 0.5 to 2.0 gsm. Typically,phenolic prepreg and plain uncoated HRH36 honeycomb have peel strengthsof around 5 inlb/3 in.

Example 19 Aircraft Interior Panel Comparison

Aircraft interior panels were made in the same manner as Example 18,except that the honeycomb was changed to a aramid paper/phenolic resincore similar to HexWeb®HRH36, except that the density of the core was3.0 pcf and it was 0.39 inch thick. The phenolic dip resin was the same.The cell size was also ⅛-inch. The prepreg that was used for the facesheets was J.D. Lincoln L-591PG-7781 having 38 weight percent resincontent. Panels were made using no edge coating and using 4.0 gsmELVAMIDE®8066 (2.0 gsm per edge) in the same manner as Example 18. Thepanel with no edge coating had an areal weight of 0.297 pounds persquare foot (psf). The panel with edge coating (4.0 gsm) had an arealweight of 0.314 psf.

With no edge coating, the peel strength of the panel at room temperaturewas 3 in-lbs/3 in width. With edge coating, the peel strength was 13in-lbs/3 in width. The Long Beam Load was 95 lbs without edge coatingand 151 lbs when the edge coating was used. Long Beam Skin Stress was19800 psi without edge coating and 31400 psi when edge coating was used.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited by the above-preferredembodiments, but is only limited by the following claims.

1. A method for making an aircraft interior or floor panel comprisingthe steps of: providing a first uncured face sheet comprising aninterior surface and an exterior surface, said first uncured face sheetcomprising at least one fiber layer and an uncured thermosetting resinmatrix; providing a second face sheet comprising an interior surface andan exterior surface; providing a honeycomb having walls that define aplurality of honeycomb cells wherein said walls have a first edge forbonding to the interior surface of said first uncured face sheet and asecond edge for bonding to the interior surface of said second facesheet; applying an adhesive only to said first edge to form anedge-coated honeycomb wherein said adhesive comprises a rubber-modifiedthermosetting resin; and bonding said uncured first face sheets and saidsecond face sheet to said edge-coated honeycomb, said bonding comprisingthe step of curing said uncured thermosetting resin matrix.
 2. A methodfor making an aircraft interior panel according to claim 1 wherein saidaircraft interior panel is selected from the group consisting ofoverhead storage bin panels, ceiling panels, bulkhead panels, lavatorypanels, wardrobe panels, galley panels, partition panels and side wallpanels.
 3. A method for making an aircraft interior panel according toclaim 2 wherein the density of said honeycomb is from 1.5 pound percubic foot to 5.0 pounds per cubic foot.
 4. A method for making anaircraft interior panel according to claim 2 wherein saidrubber-modified thermosetting resin comprises a thermosetting resinselected from the group consisting of epoxy resin, phenolic resin,cyanate esters and bismaleimides.
 5. A method for making an aircraftinterior panel according to claim 2 wherein the amount of said adhesivethat is present on said first edge of said edge-coated honeycomb rangesfrom 0.5 to 2.0 grams per square meter.
 6. A method for making anaircraft interior or floor panel according to claim 1 wherein saidsecond face sheet is an uncured face sheet that comprises at least onefiber layer and an uncured thermosetting resin matrix and wherein saidmethod includes the steps of : applying an adhesive only to said secondedge of said honeycomb wherein said adhesive comprises a rubber-modifiedthermosetting resin; and bonding said uncured second face sheet to saidedge-coated honeycomb, said bonding comprising the step of curing saiduncured thermosetting resin matrix.
 7. A method for making an aircraftinterior panel according to claim 6 wherein said aircraft interior panelis selected from the group consisting of overhead storage bin panels,ceiling panels, bulkhead panels, lavatory panels, wardrobe panels,galley panels, partition panels and side wall panels.
 8. A method formaking an aircraft interior panel according to claim 7 wherein thedensity of said honeycomb is from 1.5 pound per cubic foot to 5.0 poundsper cubic foot.
 9. A method for making an aircraft interior panelaccording to claim 7 wherein said rubber-modified thermosetting resincomprises a thermosetting resin selected from the group consisting ofepoxy resin, phenolic resin, cyanate esters and bismaleimides.
 10. Amethod for making an aircraft interior panel according to claim 7wherein the amount of said adhesive that is present on said second edgeof said edge-coated honeycomb ranges from 0.5 to 2.0 grams per squaremeters.
 11. A method for making an aircraft floor panel according toclaim 1 wherein said aircraft floor panel is selected from the groupconsisting of aisle floor panels, under seat floor panels, galley floorpanels and high load floor panels.
 12. A method for making an aircraftfloor panel according to claim 11 wherein the density of said honeycombis from 5 pounds per cubic foot to 15 pounds per cubic foot.
 13. Amethod for making an aircraft floor panel according to claim 11 whereinsaid rubber-modified thermosetting resin comprises a thermosetting resinselected from the group consisting of epoxy resin, phenolic resin,cyanate esters and bismaleimides.
 14. A method for making an aircraftfloor panel according to claim 11 wherein the amount of said adhesivethat is present on said first edge of said edge-coated honeycomb rangesfrom 0.5 to 2.0 grams per square meter.
 15. A method for making anaircraft floor panel according to claim 6 wherein said aircraft floorpanel is selected from the group consisting of aisle floor panels, underseat floor panels, galley floor panels and high load floor panels.
 16. Amethod for making an aircraft floor panel according to claim 15 whereinthe density of said honeycomb is from 5 pounds per cubic foot to 15pounds per cubic foot.
 17. A method for making an aircraft floor panelaccording to claim 15 wherein said rubber-modified thermosetting resincomprises a thermosetting resin selected from the group consisting ofepoxy resin, phenolic resin, cyanate esters and bismaleimides.
 18. Amethod for making an aircraft floor panel according to claim 15 whereinthe amount of said adhesive that is present on said second edge of saidedge-coated honeycomb ranges from 0.5 to 2.0 grams per square meter. 19.A method for making an aircraft interior panel according to claim 1wherein said rubber-modified thermosetting resin comprisesacrylonitrile-butadiene copolymer rubber.
 20. A method for making anaircraft floor panel according to claim 6 wherein said rubber-modifiedthermosetting resin comprises acrylonitrile-butadiene copolymer rubber.